Dry impact coating of powder metal parts



3,549,357 DRY IMPACT COATING OF POWDER METAL PARTS Earl H. Osborne,Colorado Springs, Colo., assignor to Allegheny Ludlum Steel Corporation,Brackenridge, Pa., a corporation of Pennsylvania No Drawing. Filed June24, 1968, Ser. No. 739,198

Int. Cl. B22f 7/02 U.S. Cl. 75208 4 Claims ABSTRACT OF THE DISCLOSUREPowder metallurgical processes are well known techniques for producingmetal articles in forms that are otherwise difiicult to manufacture.Powdered metal, such as iron and iron alloys, is used in the manufactureof many useful articles by processes in which the powders are firstcompacted by pressure alone into the approximate shape of the finishedarticle and sintered at an elevated temperature, e.g., 14002l00 F., infurnaces provided with the protective atmosphere to prevent oxidatlon.In sintering, the powders coalesce and are bonded into an integralmetallurgical structure. When desirable, the sintered atricles may beadditionally processed.

It is sometimes also desirable to provide coatings on powder metalparts. Such coatings may be applied for protection against corrosion orto achieve other special affects. The present invention provides aunique method of producing coated powder metal parts by dry impactcoating. In accordance with the invention there is provided a methodwhich comprises compacting and sintering powder metal into an article ofdesired configuration. The compacted and sintered article is thentumbled in a confined area in contact with impacting elements such asgrinding balls and in the presence of a coating material. Duringtumbling, the powder metal article is coated, and, after coating excesscoating material is removed. Thereafter the coated powder metal articleis sintered to bond the coating to the base metal. Specific sinteringtemperatures and conditions for various metals are disclosed intextbooks such as Fundamentals of Powder Metallurgy by W. D. Jones, 1961edition.

Although the method in accordance with the invention is useful inprocessing powder metal parts of various compositions, it isparticularly well adapted for producing articles of iron and ironalloys. Powder metal parts may be produced by conventional techniques inwhich iron or iron alloy powder is compacted to a medium density ofabout 5.5 grams per cubic centimeter to 7.4 grams per cubic centimeter.Small additions of copper or other metals may be added to the base metalto improve physical properties. The compacted metal powder is thensintered, usually in a conventional sintering furnace, in either a batchor continuous operation. For an iron bearing compact, a sinteringschedule of l40 minutes at a temperature of 19002150 is preferred. Thesintering temperature is, of course, a function of the material and thephysical properties required and will vary with different metals.However, the powder metallurgical arts are sufficiently well developedso that this information is well known to those skilled therein. Thecompacted and sintered iron alloy article is then placed in a tumbler,preferably substantially immediately after sintering to avoid and/orminimize contamination of the powder metal article by, for example,oxidation or soiling. The sintered article is tumbled in contact withimpacting elements such as steel balls of varying diameters.

United States Patent 0 "ice It is desirable to employ impacting elementsof different sizes to assure contact with all surfaces of the powdermetal article. Particulate coating material such as powdered copper,tin, cadmium, or zinc of high purity is added to the tumbler to coat thesintered compact. The coating material need not be of any specialparticle size or shape, but the speed of the coating process can begenerally increased by using smaller particle sizes.

It has been observed that tumbling is best performed with the impactingelements and the sintered compact experiencing a falling rather thansliding action. This may be achieved with tumbler operations of between30 and revolutions per minute. Desirably, the tumbling cycle should befrom 30 to 45 minutes, depending upon the thickness of the coatingdesired.

After tumbling to coat the powder metal article, it is removed from thetumbler and any loose powder removed from the surface. Thereafter, thecoated article is sintered, preferably in a conventional sinteringfurnace, for a time and at a temperature which is a function of themetal coating. Thus, for example, with a copper coating on an iron alloycompact, the coated article may be satisfactorily sintered in 20 to 45minutes at 1400l600 F. in a non-oxidizing atmosphere. Inert or reducinggases may be used and a hydrogen-containing gas is preferred. The objectof sintering the coated article is to metallurgically bond the coatingto the base metal. Following sin tering the coated powder article may besubsequently processed as desired. Thus, for example, the articles maybe sized, coined or impregnated in accordance with well known practices.

The following is presented as a specific example of the practice of theinvention according to the presently preferred embodiment.

Iron powder of 99% purity is blended with a die lubricant, e.g. stearicacid, for about 20 minutes. The blended powder is pressed in a die underpressure of about 30 tons/inch to form a green compact suitable forhandling. Green compacts of iron will usually be of medium density inthe range of about 5.5 gr./cc. to 7.4 gr./cc. and for the purposes ofthe invention must be less than the theoretical density. The compact istransferred to mesh belt continuous furnace and sintered at about 2050F. for about 30 minutes.

The sintered article is then placed in a rotary tumbler with metal beadsand shot. The coating material, fine copper powder of 99% purity whichis used in this example, is added to the tumbler. The tumbler is rotatedto provide an impacting rather than a sliding action of the beads andshot on to the sintered article and the relatively soft copper powder issmeared or impacted on the article to coat same. Since the sinteredarticle has a density less than theoretical, minute pores are present onthe surface and the coating material lodges into these pores orcrevices.

The copper coated article is removed from the tumbler and excess coatingmaterial is shaken off. The coated article is then sintered at atemperature below the melting point of the coating material. In thisexample, sintering would be performed by heating at about 1550 F. forabout 20 minutes. During sintering the copper is metallurgically bond tothe iron base.

It is apparent from the above that various changes and modifications maybe made without departing from the invention.

Accordingly, the scope of the invention should be limited only by theappended claims wherein what is claimed is:

1. A method of producing coated powder metal articles which comprises:

(a) compacting and sintering metal powder consisting essentially of ironand iron alloys in the configura- 4. A method according to claim 1wherein said coattion of said article, ing material is a metal from thegroup consisting of tin, (b) tumbling said article in a confined area incontact cadmium and zinc.

with impacting elements and in the presence of particulate coatingmaterial from the group consist- 5 References Cited ing essentially ofcopper, tin, cadmium, and zinc whereby said article is coated with saidcoating UNITED STATES PATENTS material, 2,251,410 8/1941 Koehring29182.2 (c) removing excess coating material from said coated 2,490,54312/1949 Robertson 29191.2X

metal article, and 3,142,559 7/1964 Ruf et al 75-208 (d) sintering saidcoated article at a temperature below 3 237 157 11/196 Brown 117 31 themelting point of a said coating material to bond 323 1 7 1 Simon saidcoating thereto. 2. A method according to claim 1 wherein the initialCARL D, QUARFORTH, Primary Examiner compacting and sintering areperformed at a temperature of 1900 F. to 2150 F. for from 10 to min-J'STEINERASslstam Exammer utes.

3. A method according to claim 1 wherein the coating material is copperpowder and the copper coated article 11731 is sintered from 20 tominutes at 1400 F. to 1600" F. 20

